Heretofore, n-propyl acetate, isobutyl acetate, n-butyl acetate, etc., as saturated esters have been widely used as dissolving agents, solvents and reaction solvents, and have become industrially important compounds. These saturated esters are generally produced via the esterification based on the condensation of the corresponding alcohol with carboxylic acid. However, in such an esterification reaction system, the equilibrium state of the reaction cannot be shifted toward the product (ester) side, unless water as a by-product is removed to the outside of the system, and therefore it is difficult to obtain industrially advantageous reaction rate and conversion of the raw material.
In order to solve this problem, there have been presented various proposals such as that in KOKAI (Unexamined Patent Publication) No. Hei. 5-194318. However, as described in this KOKAI publication, there is a problem in that a more complicated reaction apparatus and reaction process are required for the process for industrially producing the carboxylic acid ester via the esterification of the alcohol with the carboxylic acid, as compared with those processes using the other reaction systems.
Further, in the esterification reaction based on the condensation of the alcohol with the carboxylic acid, water is inevitably produced in the system. However, the latent heat of vaporization of water is much larger than the latent heats of vaporization of other organic compounds, and therefore there is also a difficulty in that much energy is consumed in separating the water by distillation.
On the other hand, an unsaturated group-containing ester which contains an unsaturated group such as allyl group, methallyl group, and vinyl group at the alcohol site of the ester can be industrially produced, e.g., via the oxidative carboxylation reaction of the corresponding alkene with carboxylic acid, etc.
Particularly, it is well known that an unsaturated group-containing ester can be produced by reacting the corresponding alkene, oxygen and carboxylic acid in a gas phase in the presence of a palladium catalyst, and there are many known documents concerning such a process. Examples thereof include KOKOKU (Examined Patent Publication) No. Sho. 44-29046, KOKOKU No. Sho. 48-23408, KOKOKU No. Sho. 50-28934, and KOKAI No. Hei. 1-197457. Among these, KOKAI No. Hei. 1-197457 discloses that an unsaturated group-containing ester can be industrially produced at a very high yield and a high space-time yield through the oxidative carboxylation of the corresponding olefin with carboxylic acid.
On the other hand, it is said that an allyl-type ester as one kind of the above-mentioned unsaturated group-containing ester can be industrially produced, e.g., via the above oxidative carboxylation reaction of the corresponding alkene with carboxylic acid, as well as via the reaction of an allyl-type chloride with a carboxylic acid or a carboxylic acid salt, the esterification reaction based on the condensation of an allyl-type alcohol with a carboxylic acid, etc.
Accordingly, various methods, wherein these easily available unsaturated group-containing esters such as allyl-type ester are subjected to a hydrogenation reaction so as to produce a corresponding hydrogenated ester (particularly, saturated ester), have been investigated.
It has heretofore been known that a metal selected from Group 8 elements, Group 9 elements, and Group 10 elements in the periodic table (according to Nomenclature of Inorganic Chemistry, Revised Edition, 1989, International Union of Pure and Applied Chemistry; and in the same manner in the description hereinafter) is effective as a hydrogenating catalyst to be used for the hydrogenation reaction of an unsaturated group-containing ester. However, there has been pointed out the hydrogenolysis of the unsaturated group-containing ester as the raw material, as a problem encountered in the method using such a metal-type catalyst. More specifically, it is known that when an allyl-type ester represented by the following formula (1) or an enol-type ester represented by the following formula (2) is hydrogenated in the presence of a catalyst containing a metal selected from Group 8 elements, Group 9 elements, and Group 10 elements to thereby produce a saturated ester, the hydrogenolysis of the unsaturated group-containing ester as the raw material occurs so as to produce the corresponding carboxylic acid and alkane as by-products (“Catalytic Hydrogenation-Applications to Organic Synthesis” (1st edition, 1st printing, issued on Apr. 10th, 1987, Tokyo Kagaku Dojin), page 116 et seq., may be referred to). 
As a measure for solving such a problem, there is known a method wherein palladium metal is used as a catalyst in view of the suppression of the hydrogenolysis of the enol-type ester (the above-mentioned “Catalytic Hydrogenation-Application to organic Synthesis”, page 116 et seq., may be referred to). Further, there is known a method wherein a rhodium metal is used as a catalyst so as to suppress the hydrogenolysis of an allyl-type ester (W. F. Berkowitz, I. Sasson, P. S. Sampathkumar, J. Hrable, S. C. Choudhry, D. Pierce, Tetrahedron Lett, 1979, page 1641).
However, according to the present inventors' experiments, it has been found that each of the above-mentioned catalysts can effectively suppress the hydrogenolysis with respect to either one of the enol-type ester or the allyl-type ester, but each of the catalysts cannot effectively suppress the hydrogenolysis of both of these unsaturated group-containing esters.
On the other hand, KOKAI NO. Hei. 9-194427 discloses a method for producing a saturated ester from an unsaturated group-containing ester by use of a nickel-type hydrogenating catalyst. According to this patent publication, it is said that this method can effectively suppress the hydrogenolysis by using the nickel-type hydrogenating catalyst, as compared with the method using a palladium metal as a catalyst and the method using a rhodium metal as a catalyst.
However, when the present inventors have tried to practically carry out the hydrogenation reaction according to the above method of the patent publication so as to produce the corresponding saturated ester, it has been found that this process is also accompanied with the decomposition reaction to the corresponding alkane and the corresponding carboxylic acid (i.e., the hydrogenolysis of the raw material), at least to a certain extent.
More specifically, according to the present inventors' experiments, it has been found that even when the above method according to KOKAI No. Hei. 9-194427 is used, it is difficult to conveniently obtain a saturated ester having a high purity reaching the product standard for the saturated esters which are widely used as solvents and reaction solvents. That is, in general, according to the product standard for the saturated esters, the tolerable amount of carboxylic acid to be contained therein corresponds to a concentration of 50 ppm or less, and the purity of the saturated ester is 99.5 mass % or more. In other words, the production of such a carboxylic acid due to the hydrogenolysis reaction poses a very serious problem in view of the product quality of the saturated esters. In addition, it is generally difficult to separate an unsaturated group-containing ester as a raw material (e.g, allyl acetate) and a saturated ester as a product (e.g., n-propyl acetate) from each other by a simple distillation, and therefore a precise multi-stage distillation is required in order to achieve such a product standard.
In view of the foregoing, and in order to simplify the purification step after the completion of the reaction, it is extremely preferred that the conversion of the unsaturated group-containing ester is made 99.8% or more, the selectivity factor for the carboxylic acid produced as a by-product is made as low as possible (the selectivity factor for the carboxylic acid may preferably be 2.0% or less), and the selectivity factor for the hydrogenated ester (particularly, that for the saturated ester) is made higher (the selectivity factor for the saturated ester is made 98.0% or more).
However, for example, the above-mentioned patent publication KOKAI No. Hei. 9-194427 does not describe an En achievement such that when a saturated ester is produced from an unsaturated group-containing ester by use of a nickel-type hydrogenating catalyst, the conversion of the unsaturated group-containing ester is made 99.8% or more, and the selectivity factors for the saturated ester and the carboxylic acid are made 98.0% or more and 2.0% or less, respectively.
An object of the present invention is to provide a process for efficiently producing a hydrogenated ester, wherein the formation of a carboxylic acid due to the hydrogenolysis as a side reaction to the hydrogenation reaction is suppressed when the corresponding hydrogenated ester is produced by the hydrogenation reaction of an unsaturated group-containing ester.